U.S. Pat. No. 6,199,021, and related C.I.P. U.S. Pat. No. 6,356,848 describe an apparatus and method for measuring the vibrational frequency of an elongate flexible member such as a bicycle chain. In such a system, a magnetic coil is used to measure the vibrational frequency of the chain. One obstacle that must be overcome with this system is an interference signal caused by the linear motion of the chain links moving past the magnetic coil sensor. This signal can obscure the vibrational frequency signal of the chain, where the vibrational frequency is directly related to the tension carried by the chain. Patents '021 and '848 disclose various methods to differentiate and filter these two signals.
The magnetic coils used in the inventive system are similar to the coils used in other applications, including electric guitars. Guitar applications must overcome a different signal interference issue, where the coil detects not only the desired signal of the vibrating strings, but also a 60 Hz signal from electrical supply lines. Guitars have solved this problem by using two separate coils, wound in opposite directions, which effectively filters the 60 Hz noise through destructive interference. This dual coil arrangement is commonly referred to as a humbucker, with variations of the system also used.
While a humbucker system is in some respects analogous to the inventive device, there are, however, significant differences in the systems. With a guitar, the interference signal is generated from an outside source. But in the application of the inventive device, the chain itself generates several different frequencies, mainly due to the chain being a non-homogonous body. In addition, the inventive device works in conditions where these frequencies are constantly varying, unlike a constant 60 Hz interference signal.